A system of this kind is disclosed in U.S. Pat. No. 5,181,211 to Burnham et al, which works from the reported understanding that laser beams having a wavelength in the range of 1.5 .mu.m to 2.2 .mu.m are completely absorbed by the vitreous humor of the eye cornea, thereby avoiding any damage to the retina. The patent proposes the combination of a solid state laser, such as a diode-pumped neodymium laser, with a non-linear crystal parametric converter in an associated resonant optical cavity. In order to ensure the highest efficiency of operation, the patent requires non-critical phase matching in that the incident beam from the laser is transmitted parallel to one of the principal axes of the converter crystal. The output beam is said to be polarised in a direction parallel to a second of the principal axes and the idler beam is polarised in a direction parallel to the third principal axis. A preferred non-linear crystal is potassium titanyl phosphate (KTP).
The present invention stems from an appreciation that, contrary to U.S. Pat. No. 5,181,211, beam propagation parallel to a principal axis is not necessary and that beam propagation at an angle with .+-.10.degree. with respect to a principal axis can achieve parametric oscillation with no significant degradation of efficiency. Beam propagation parallel to a principal axis is also very difficult to align. Moreover, if the beam propagation is specifically off-axis, the output wavelength of the parametric converter can be tuneable according to the offset angle. This offers much greater flexibility than the type proposed in U.S. Pat. No. 5,181,211.